# Frank Solutions and MCQ for Chapter 3 Sound Class 10 Physics ICSE

## Exercises

1. What is meant by echo? What are the conditions necessary for its formation?
The sound heard after reflection from a rigid obstacle (such as cliff, a hillside, a wall of a building, edge of a forest etc.), is called an echo.
Conditions necessary for echo formation are:
1. The minimum distance between the source of sound and its reflector should be 17 m.
2. Reflected sound should reach the person atleast 0.1 second after the original sound is heard.

2. Why do echoes produced in an empty auditorium usually decrease when it is full of audience?
The audience in the auditorium as act obstructions in the path of sound waves; so they sound waves are not reflected much less and hence the echoes usually decrease.
4. A girl claps and hears the echo after reflection from a cliff which is 660 m away. If the velocity of sound is 330 ms-1, calculate the time taken for hearing the echo.

5. A rifle shot is fired in a valley between two parallel walls. The echo from one wall is heard in 3 s and the echo from the other wall is heard 3 s later. If the velocity of sound at 00c is 330 ms-1 and the temperature in the valley is 100c, calculate the width of the valley. For every 10c rise in temperature, the velocity of sound increase by 0.61 ms-1.
Give that, speed of sound at 0° = 330 m/s
For every 1° rise in temperature, the velocity of sound increase by 0.61 m/s
∴ The velocity of sound at 10° = 330+(0.61×10) = 330 +6.1 = 336.1 m/s
Let A1 and A2 be the distances of the two walls from the point of rifle shots. Then
2×A1 = 336.1×3 = 1008.3 ....(i)
2×A2 = 336.1 × 6 = 2016.6 ....(ii)
Adding (i) and (ii) we get :
2(A1 + A2 ) = 3024.9
or, (A1 + A2 ) =1512.45 m
Thus, the width of the valley is 1512.45 m

6. Explain how is the principle of echo used by
(a) The bat during its flight at night,
(b) The dolphin to locate small fish as its prey.
(a) Bats can produce and detect the sound of very high frequency. The bats fly with speed much lower than the speed of sound. The sounds produced by flying bats get reflected from any obstacle in front of it. By hearing the echoes, bats come to know where the obstacles are, even in the dark. So, they can fly safely without colliding with the obstacles. This process of detecting obstacles is called sound ranging.
(b) Dolphins detect their enemies and small fishes by emitting ultrasonic waves in all directions and then hearing their reflected sound i.e. echo. Dolphins can judge the nature of obstacles or of small fish by hearing the echo and catch their prey.

7. Explain the following and illustrate your answer by drawing appropriate diagrams:
(a) Frequency,
(b) Loudness,
(c) Pitch,
(d) Quality of a musical note.
(a) Frequency: Frequency of a vibrating body is defined as the number of vibrations completed by the body in one second. (b) Loudness: Loudness is the property by virtue of which a loud sound can be distinguished from a faint one, both having the same pitch and quality. The loudness of a sound depends on the amplitude (or intensity) of the wave. (c) Pitch: The 'pitch' of a note is determined solely by its frequency. The pitch in fact, is a subjective sensation in the ear depending only upon the frequency of the musical note. The relation between pitch and frequency is linear to a very close approximation. (d) Quality of a musical note: quality of a sound is that characteristic which distinguishes the two sounds of the same loudness and same pitch, but emitted by two different instruments. It depends on the waveform.

8. Define the terms velocity and wavelength applied to sound wave and state the relation between velocity, frequency and wavelength.
Wave- velocity: It is the velocity with which a wave propagates in a particular medium.

Wavelength: The wavelength is the distance between two successive crests or two successive troughs on a transverse wave.
It is also equal to the distance between any two points where the particles are passing through their respective mean positions in the same direction.
It is also the distance between two successive compressions or two successive rarefactions on a longitudinal wave.

Wave-velocity = wavelength × frequency

9. What is the wavelength of sound waves produced in air by a vibrating tuning fork whose frequency is 256 Hz when the velocity of sound in air is 330 ms-1?

Given, velocity = 330 m/s, frequency = 256 Hz
∴ Wave - length = velocity/frequency = 330/256 = 1.3 m

10. State the characteristics of a musical note on which its pitch, loudness and quality depend.
Pitch of a musical noted depends on its wavelength or frequency. Higher the frequency, higher the pitch.
Loudness depends upon the amplitude of the wave. Loudness is directly proportional to the square of amplitude.
Quality of a musical depends on the wave form.

11. In fig. 1 shows a snap shot of a wave from of frequency 50 Hz in a string. The numbers in the diagram represents distance in centimeters. Fpr the wave motion, find
(a) Wavelength,
(b) Amplitude,
(c) Velocity.
(a) Wavelength = Distance between two successive crests or two successive troughs = 10 cm
(b) Amplitude = Maximum displacement of the particles from the mean position = 4 cm
(c) Velocity = frequency × wave length
Given, frequency = 50 Hz,
wavelength = 10 cm = 0.1 m
∴ Velocity = 50× 0.1 = 5 m/s

12. Differentiate between the following:
(a) Light and sound waves.
(b) Free and forced vibrations.
(c) Radio waves and light waves.
(a) Difference between Light and Sound Waves
 Light waves Sound waves 1 These are electromagnetic waves.2. They can travel in vacuum.3. The speed of light waves is very high (= 3 x 108 m/s in air).4. The wavelength of the light waves (visible) is very small, of the order of 104 m.5. These waves are transverse. 1 These are mechanical waves.2. They require a material medium for propagation.3. The speed of sound waves is low (=330 m/s in air).4. The wavelength of sound waves is in the range of 10-2 m to 10m.5. These waves are longitudinal waves

(b)  Difference between Free vibrations and Forced vibrations

 Free vibrations Forced vibrations 1. When a body vibrates with its natural frequency and time period which is characteristic of the body itself, it is said to have free vibrations.2. It is difficult to realize such vibrations in practice. They can take place only in vacuum. 1. When a body is set into vibrations with the help of a strong periodic force then the vibration is said to be forced vibration. The frequency of such vibrations may not be equal to the natural frequency of the body.2. Such vibrations can be realized in practice.

(c)  Difference between Radio waves and Light waves

 Radio waves Light waves 1. Radio waves are not visible to human eye.2. These waves range from 105 to 10-3 meters.3. Frequency of radio waves is below 107 Hz.4. Its sources are TV and radio transmitters. 1. Light waves are visible to human eye.2. These waves range from 10-6 to 10-7 meters.3. Frequency of light waves lie in the range 1014 to 1013 Hz.4. Its sources are sunlight, white hot bodies

13. Write down the factors on which the frequency of vibrating depends. Explain them.
Frequency of vibrations of a stretched string depends upon:
1. Frequency of the fundamental note of a stretched string is inversely proportional to the length of the vibrating string.
2. Frequency is directly proportional to the square root of the tension of the string.
3. Frequency is inversely proportional to the square root of linear density. That is, mass per unit length of the material of the string. Thinner is the wire, higher is the frequency.

14. What is meant by resonance? Give two examples of resonance from daily life.
When the frequency of the forced vibration is equal to the natural frequency of a body nearby or an integer multiple of it then the body vibrates with a large amplitude. This phenomenon is called resonance.
Example 1: All stringed instruments are provided with sound box (or sound chamber). This box is so constructed that the column of of air inside it, has a natural frequency which is the same as that of the strings stretched on it, so that when the strings are made to vibrate, the air column inside the box is set to forced vibrations. Since the sound box has a large area, it sets a large volume of air into vibration of the same frequency as that of the string. So, due to resonance, a loud sound is produced.
Example 2: Radio and TV receivers have electronic circuits which produce electrical vibrations, the frequency of which can be changed by changing the values of the electrical components of that circuit. When we want to tune a radio or TV receiver, we merely adjust the values of the electronic components to produce vibrations of frequency equal to that of the incoming radio waves which we want to receive. When the two frequencies match, due to resonance, the energy or signal of that particular frequency is received from the incoming waves. The signal is then amplified in the receiver set.

15. Give the physical explanation for the difference between musical sounds and noises.

 Music Noise 1. It is pleasant, smooth and agreeable to the ear. 2. It is produced by the vibrations which are periodic.3. All the component waves are similar without any sudden change in their wavelength and amplitude. 4. The sound level is low (below 30 dB). 1. It is harsh, discordant and displeasing to the ear. 2. It is produced by an irregular succession of disturbances.3. The component wave changes their character suddenly and they are of short duration. 4. The sound level is high (above 120 dB).

16. Name musical instrument, one in each case, which produces its notes by using the following. In each case state the method used to produce notes of different pitches and loudness.
(a) A vibrating string.
(b) A vibrating column air.
(c) Vibration of any other body.
(a) Guitar produces notes using vibrating string.
To produce notes with different pitches, strings of different thicknesses are plucked. When a string of greater thickness is plucked, a note with higher pitch is produced and vice-versa.
To produce the notes of different loudness, all stringed instruments are provided with hollow sound box which contains air. In these instruments vibrations are produced in the sound box when the strings on it are made to vibrate by plucking, are forced vibrations. Larger the surface area of the air in the sound box, louder will be the sound produced.
(b) Flute produces notes using a vibrating column of air.
In a flute, the notes of different frequencies or pitch are produced by changing the effective length of the air column when different holes in it are closed.
In a flute, the notes of different loudness can be produced by using flutes of different diameters. A flute with larger diameter shall have more air enclosed in it and hence the sound produced by it shall be louder.
(c) Piano produces notes using a vibration of any other body.
When we strike the keys of piano, strings of different thickness are set in vibration at their natural frequencies; hence sound of higher pitch can be produced by striking the string of greater thickness.
A piano's wires are attached to a sounding board with help of which sound of different loudness can be produced. The vibrating wire makes the board vibrate, which makes the sound louder.

17. Give scientific reasons for the following:
(a) When a nail is hammering appears to get higher and higher in pitch.
(b) Windows sometimes rattle when the low notes of a pipe organ are sounded.
(a) When a nail is hammered into a piece of wood, its length outside wood gradually decreases. As the length decreases, the frequency of vibrations increases and hence the pitch being directly proportional to the frequency also increases
(b) When the windows rattle, at that moment, its natural frequency corresponds with the frequency with which the low notes of a pipe organ are sounded. Thus, resonance takes place which makes the windows to vibrate violently.

18. What is the safe limit of sound level of db for our ears?
Sound of level below 120 dB is safe for our ears.

19. What is the sound level for noise pollution?
The sound level above 120 dB causes noise pollution.

20. What determines the pitch of a sound?
The 'pitch' of a note is determined solely by its frequency. The pitch in fact, is a subjective sensation in the ear depending only upon the frequency of the musical note. The relation between pitch and frequency is linear to a very close approximation.

21. Name the subjective property of sound related to its frequency.
Subjective property of sound related to its frequency is 'pitch'.

22. How is it possible to recognize a person by his voice without seeing him?
It is possible to recognize a person by his voice without seeing him because the vibrations produced by the vocal chord of each person have a characteristic waveform which is different for different persons.

23. How does the wave from of a loud note differ from a soft note?
The loudness of a sound wave is determined by its amplitude; it is proportional to the square of the amplitude. Therefore, the louder sound corresponds to the wave of larger amplitude.

24. Why is the loudness of the sound heard by a plucked wire increased when it is mounted on a sound board?
When the strings are made to vibrate by plucking, the vibrations get transferred to the hollow sound box. As a result, the large volume of air in the box is also set into vibration to produce a loud sound. Larger the surface area of the air in the sound box, louder will be the sound produced.

25. Define the term intensity of a sound wave. State the unit in which it is measured.
The intensity of a sound wave at any point of the medium is measured as the amount of sound energy passing per second normally through unit area at that point.
Its unit is microwatt per metre2.

26. How is loudness of sound related to the intensity of wave producing it?
Loudness of a sound (L) and its intensity (I) are related as:
L = k log (I/Io),
Here, I/Io is the intensity level of the sound or the ratio between its intensity I and the threshold intensity Io, and k is the constant of proportionality depending upon the unit chosen.

27. What change do you expect in the characteristics of a musical sound if
(i) Its frequency is increased,
(ii) Its amplitude is increased?
(i) If frequency of a musical sound is increased, its pitch will increase.
(ii) If amplitude of a musical sound is increased, its loudness will also increase.

28. Comment on the statement 'loudness of sound is a subjective quantity, while intensity is an objective quantity.'
Loudness depends on the energy conveyed by the wave near the eardrum of a listener. Loudness being a sensation also depends upon the sensitivity of the ears of listener. Thus, loudness of a sound of given intensity may differ from listener to listener.
Further, two sounds of the same intensity, but of different frequencies may differ in loudness even to the same listener because the sensitivity of the ears is different for different frequencies. Thus loudness is a subjective quantity, while intensity, being a measurable quantity, is an objective quantity for a sound wave.

29. State three factors on which loudness of sound heard by a listener depends.
The loudness of a sound heard by a listener depends upon:
1. Loudness is directly proportional to the square of the amplitude.
2. Loudness varies inversely as the square of distance.
3. Loudness is directly proportional to the surface area of a vibrating body.

30. Name the unit used to measure the sound level.